Early wireless voice telephony systems utilized circuit-switched technology for both analog and digital data transfer, wherein a logical traffic channel was dedicated to each voice communication session, or call. Modern systems, in contrast, are all digital and employ packet-switched technology. In packet-switched systems, common logical traffic channels carry a plurality of data packets, each packet intended for, and addressed to, a particular mobile terminal. For secure communications, the packets containing digitally encoded speech are encrypted, such as using the Advanced Encryption Standard (AES). The encryption process adds some overhead to the data packets, which in the case of large packets, is negligible.
Human speech is not continuous, but rather includes many pauses, such as between thoughts or sentences, and when a user pauses to listen to the other party. While it would minimize the use of air interface resources to simply not transmit any data during speech pauses, experience indicates that complete silence during a pause in speech is disconcerting to users. Users prefer to hear some sound, such as background noise of the other party's environment, during pauses in speech. Accordingly, when speech is digitally encoded, “silence parameters” are encoded and transmitted in speech data frames that allow background noise to be reconstructed by a receiver. The reconstructed background noise is known in the art as “comfort noise.” The data frames carrying silence parameters are small compared to data frames carrying encoded speech.
As mentioned above, the overhead added by encryption to data packets containing encoded speech data frames is small or negligible. However, the encryption overhead is a significant portion of data frames carrying encoded silence parameters. This overhead reduces the available bandwidth, and contributes to network congestion during periods of heavy voice telephony use.